Wrap spring clutches are well known in the art and are used in a variety of forms for transmitting rotating shaft power. As illustrated in FIG. 1, the basic design of a wrap spring clutch 1 involves a coil spring 3 surrounding two shafts (or hubs) to transfer torque from the input or drive shaft 5 to an output or driven shaft 7 through the spring as the spring wraps down on both shafts. Commonly the spring is fixed on one end to one of the shafts. When the clutch is activated, the unfixed end of the spring attaches to the other shaft and is rotated until the spring is wrapped down onto both shafts, thereby allowing torque to be transmitted from one shaft to the other.
A wrap spring clutch may be activated in various ways. U.S. Pat. No. 6,247,569 describes one arrangement commonly referred to as a standard logic wrap spring clutch. In a standard logic wrap spring clutch, the resting or free inside diameter of the spring is somewhat larger than the outside diameter of the shafts, so that in the absence of some form of engaging action, the free end of the spring does not grip its respective shaft and the clutch does not transfer torque between the shafts. Thus, when no power is provided to a standard logic wrap spring clutch, no torque is transferred through the clutch. To engage the standard logic wrap spring clutch of U.S. Pat. No. 6,247,569, an electromagnet is used to connect the free end of the spring with the drive shaft, thereby causing the spring to wrap down on both shafts. A standard logic wrap spring clutch of this design requires electrical power in order to transmit torque, which is a disadvantage in some applications.
Reverse logic wrap spring clutches are also known. The resting or free inside diameter of the coil spring in a reverse acting wrap spring clutch is somewhat smaller than the outside diameter of the shafts, so that in the absence of some form of disengaging action, both shafts are gripped by the spring and torque is transmitted through the clutch. Thus, the reverse logic wrap spring clutch design eliminates the need for electrical power to transmit torque. To disengage a reverse logic wrap spring clutch, power is applied to magnetically pull a control collar which is connected to the input end of the spring against the clutch housing, thereby causing the spring to unwrap as the driven shaft continues to rotate, eventually causing the spring inside diameter to increase sufficiently to disconnect the two shafts from each other. However, operation of this design requires there to be an adequate degree of momentum in the rotating driven shaft in order to unwrap the spring. For low speed applications or when there is a low momentum driven load or a high degree of output drag, there may be insufficient continued motion of the driven shaft to completely unwrap the spring. Such incomplete disengagement of the clutch can cause excessive wear, heat and premature failure of the device.